1. Field of the Invention
The present invention relates to a primary radiator provided in a satellite broadcasting reflective antenna and the like, and more particularly, to a primary radiator in which a dielectric plate serving as a 90-degree phase shifter is placed inside a waveguide.
2. Description of the Related Art
FIGS. 9A and 9B are a left side view and a sectional view, respectively, showing such a type of conventional primary radiator. The conventional primary radiator comprises a waveguide 10 which is opened at one end and is closed at the other end, a dielectric plate 11 placed inside the waveguide 10, and a pair of probes 12 and 13 inserted in the waveguide 10 through outer wall surfaces thereof. The probes 12 and 13 are disposed at a distance corresponding to about one quarter the guide wavelength from the closed surface of the waveguide 10. The waveguide 10 is a rectangular waveguide having a cavity of rectangular cross section. Although not shown, a horn portion is formed at the open end of the waveguide 10 so as to receive electric waves. Such a rectangular waveguide has, for example, the advantage of reducing the area of a printed circuit board (not shown) connected to the probes 12 and 13, compared with a circular waveguide of circular cross section. The dielectric plate 11 functions as a 90-degree phase shifter, and is made of a dielectric material having a uniform thickness. The dielectric plate 11 is fixed to both diagonal corners of the waveguide 10, and both ends thereof in the longitudinal direction are cut out in a V-shape in order to improve the input impedance and output impedance. The probes 12 and 13 are orthogonal to each other, and the dielectric plate 11 is disposed at an angle of approximately 45xc2x0 to the probes 12 and 13.
In the primary radiator with such a configuration, for example, in order to receive a right-handed circularly polarized wave and a left-handed circularly polarized wave transmitted from a satellite, the circularly polarized waves are guided into the waveguide 10 from the open end via the horn portion (not shown), and are converted into linearly polarized waves inside the waveguide 10 by the dielectric plate 11. That is, since a composite vector of two linearly polarized waves having the same amplitude and having a 90-degree phase difference therebetween rotates in a circularly polarized wave, when the circularly polarized wave passes through the dielectric plate 11, the phases shifted 90xc2x0 are caused to become the same phase and the circularly polarized wave is converted into a linearly polarized wave. Since the left-handed circularly polarized wave is converted into a vertically polarized wave and the right-handed circularly polarized wave is converted into a horizontally polarized wave in the example shown in FIGS. 9A and 9B, by receiving the vertically polarized wave and the horizontally polarized wave after coupling the waves to the probes 12 and 13, the received signals can be subjected to frequency conversion by a converter circuit (not shown) and can be then output as IF signals.
In the primary radiator with the above-described configuration, the electric field distribution inside the waveguide 10 of rectangular cross section is shown in FIG. 10. This figure shows that an electric field E1 (shown by broken lines) and an electric field E2 (shown by solid lines) have an intensity distribution such as to spread in an arc-shaped form from the corners of the waveguide 10 and that the electric field E1 does not exist at both ends of the dielectric plate 11 fixed to the corners of the waveguide 10. This is because the electric fields E1 and E2 are directed perpendicularly to the flat surfaces of the waveguide 10, and as a result, polarized wave components propagating through the dielectric plate 11 are reduced. For this reason, in order to cause the phases shifted 90xc2x0 to become the same phase by the dielectric plate 11, the dielectric plate 11 must be sufficiently long along the center axis of the waveguide 10. That is, the required length of the circularly polarized wave converting section is increased, and this inhibits the size reduction of the primary radiator.
By fixing the dielectric plate 11 perpendicularly to the opposing flat surfaces of the waveguide 10, polarized wave components propagating through the dielectric plate 11 are increased. In this case, since the probes 12 and 13 disposed at approximately 45xc2x0 with respect to the dielectric plate 11 must be placed at the corners of the waveguide 10, no electric field exists around the probes 12 and 13, and this makes it impossible to couple the linearly polarized waves converted by the dielectric plate 11 to the probes 12 and 13.
The present invention has been made in view of the circumstances of the conventional art, and an object of the invention is to provide a primary radiator which is suitably reduced in size by shortening a dielectric plate serving as a 90-degree phase shifter.
In order to achieve the above object, according to an aspect of the present invention, there is provided a primary radiator including a first waveguide having a rectangular opening at one end, a dielectric plate placed inside the first waveguide so as to be substantially orthogonal to two parallel sides of the opening, a second waveguide of rectangular cross section coaxially connected to the other end of the first waveguide, and a probe protruding from an inner wall surface of the second waveguide toward the center axis, wherein the inner wall surface of the second waveguide is disposed at an angle of approximately 45xc2x0 with respect to the dielectric plate.
In the primary radiator with such a configuration, the dielectric plate placed inside the first waveguide is disposed at an angle of approximately 45xc2x0 with respect to the flat surface of the second waveguide and is substantially orthogonal to two parallel sides of the opening of the first waveguide. Therefore, even when the length of the dielectric plate is reduced, the phase difference with respect to orthogonal polarized waves is increased, and the size of the primary radiator can be reduced. In this case, while it is preferable that the opening of the first waveguide be shaped like a regular square, it may be shaped like a regular polygon having two opposing parallel sides, such as a regular hexagon or a regular octagon.
According to another aspect of the present invention, there is provided a primary radiator including a first waveguide having a circular opening at one end, a dielectric plate placed inside the first waveguide, a second waveguide of rectangular cross section coaxially connected to the other end of the first waveguide, and a probe protruding from an inner wall surface of the second waveguide toward the center axis, wherein the inner wall surface of the second waveguide is disposed at an angle of approximately 45xc2x0 with respect to the dielectric plate.
In the primary radiator with such a configuration, the dielectric plate placed inside the first waveguide is also disposed at an angle of approximately 45xc2x0 with respect to the flat surface of the second waveguide, and the phase difference with respect to orthogonal polarized waves is increased even when the length of the dielectric plate is reduced. This can reduce the size of the primary radiator.
In the above configurations, it is preferable that a corner between adjoining inner wall surfaces of the second waveguide be inscribed in the opening of the first waveguide. In this case, the first waveguide and the second waveguide connected in the axial direction can be easily produced by extending a part of a waveguide of rectangular cross section by rolling.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.